Liquid refrigerant condensed inside refrigerant channels occupies an insignificant part of the entire internal condenser volume, but it sticks to the condenser walls and covers up significant part of its heat transfer area. As a result, vapor refrigerant, which occupies a significant part of the entire internal volume, does not contact the condenser walls and overall heat transfer ratio is substantially reduced.
A number of patents (U.S. Pat. No. 5,988,267 and U.S. Pat. No. 5,762,566) addressed this by splitting condensers in a number of passes and removing of a condensed portion from refrigerant stream after each pass. This reduces mass flow rate in each following pass, increases the heat transfer area interfacing with the condensing vapor, improves overall heat transfer ratio, reduces temperature difference required for the condenser duty, and reduces discharge pressure. As a result, performance characteristics are enhanced if heat transfer area parity is provided or the cost is reduced if parity of the performance characteristics is provided.
Such condensers may provide substantial sub-cooling in the last condensation pass only. When liquid refrigerant streams outgoing from all condenser passes are mixed, liquid sub-cooling of the entire refrigerant stream is reduced. If the liquid line is too long and/or pressure drop in the liquid line is substantially high, then at certain operating conditions there is potential risk of evaporation of liquid refrigerant at the expansion device inlet. Evaporation of liquid refrigerant at the expansion device inlet results in unstable operation of the entire refrigerating system and degradation of performance characteristics.
In U.S. Pat. No. 5,752,566 a condenser has a plurality of headers having baffles and/or phase separators positioned therein. The refrigerant strikes a sidewall of one of the headers and respective phases are separated by gravity. Additionally, phase separators may be used to selectively route the vapor and liquid phases to specific locations in the condenser. This patent implies that refrigerant after the condenser is directed to a liquid refrigerant receiver and then to a sub-cooling section. The sub-cooling section handles the entire refrigerant mass flow rate and carries thermal load associated with the entire refrigerant mass flow rate. Since the temperature difference driving the heat transfer process is significantly lowered, the sub-cooling section may be classified as an inefficient heat exchanging device in comparison with the condensation section.
U.S. Pat. No. 6,385,981 B1 relates to refrigerating systems accommodating the economizing cycle. The economizing cycle utilizes an economizing heat exchanger providing thermal contact between liquid refrigerant in the liquid line and evaporating refrigerant at a pressure lower than the discharge pressure and higher than the suction pressure. Such a heat exchanger has substantially high overall heat transfer ratio between liquid and evaporating refrigerant streams and, therefore, provides very efficient sub-cooling duty. This significantly reduces initial cost of means providing the adequate sub-cooling. However, refrigerating systems utilizing the economizing cycle require increased refrigerant mass flow rate through the condenser and, therefore, demand higher condenser capacities and sizes. Additionally, they elevate potential risk of evaporation of liquid refrigerant at the expansion device inlet.
U.S. Pat. No. 5,692,389 relates to refrigerating systems accommodating the economizing cycle with a flash tank. The flash tank has vapor and liquid outlets. The liquid outlet feeds a circuit with an evaporator. The vapor outlet feeds a circuit with the economizer inlet at a pressure lower than the discharge pressure and higher than the suction pressure. The flash tank provides liquid refrigerant at the liquid outlet at the same temperature as an economizing heat exchanger does in the above-mentioned example. The refrigerating systems utilizing an economizing cycle with a flash tank require increased refrigerant mass flow rate through the condenser, demand higher refrigerant mass flow rate through the condenser, higher condenser capacities and sizes than refrigerating systems utilizing an economizing cycle with an economizing heat exchanger.